This invention relates to microwave heating apparatus. It relates more particularly to microwave apparatus including a catheter capable of being introduced into a patient""s blood vessel or body cavity to provide uniform and controlled heating of fluid or tissue within the patient. The invention has particular application as an intravascular blood warmer for raising the body core temperature of a hypothermic trauma patient and so the invention will be described primarily in that context. It should be understood, however, that aspects of the invention have equal application in other contexts such as benign prostatic hyperplasia (BPH) ablation and myocardial ablation.
Hypothermia in trauma patients (i.e., body core temperature less than 35xc2x0 C.), has been shown to be associated with high mortality. According to studies, trauma patients having a temperature less than 34xc2x0 C. have a 60% mortality and those patients with a temperature less than 32xc2x0 C. have a 100% mortality. The effects of hypothermia on trauma patients are numerous. For example, a decrease in core temperature results in decreased mental status, decrease heart rate and cardiac output and diminished renal blood flow.
Hypothermia also results in prolonged clotting times and portal sequestration of platelets causing peripheral thrombocytopenia as well as decreased platelet finction. The resultant coagulopathy may make futile all attempts at surgical control of traumatic bleeding.
There are currently several methods of rewarming a trauma patient in general use today. These include use of warm resuscitation fluids, airway rewarming, heating blankets, overhead radiant warmers, body cavity lavage, continuous arteriovenous rewarming (CAVR) and cardiopulmonary bypass. The most effective method of rewarming is currently cardiopulmonary bypass, but this technique is often unavailable and is technically difficult to perform. CAVR has been shown to be,much more efficient than other standard rewarming techniques, but it requires cannulation of both the femoral artery and vein in order to connect the patient to a conventional external heat exchanger and it is somewhat work intensive. Furthermore, it results in loss of blood because a considerable amount of fluid is required in order to fill up or prime the various IV tubes connected to the warmer.
It would be desirable, therefore, to be able to provide a simple, efficient means of rewarming trauma patients, especially soldiers in combat who are significantly injured and therefore at risk for developing hypothermia controlled intracorporeal heating for other reasons is also a desirable objective.
Accordingly, it is an object of the present invention to provide an improved technique for controlledly heating fluid or tissue in a patient.
Another object of the invention is to provide microwave warming apparatus for efficiently heating or rewarming fluid or tissue in a patient.
A further object of the invention is to provide such apparatus which provides uniform and controlled heating in a hypothermic trauma patient.
Another object is to provide such warming apparatus which requires only a single venous connection to a patient and which minimizes patient blood loss.
An additional object of the invention is to provide an improved intravascular microwave warming catheter which minimizes blockage of, and injury to, the blood vessel in which it is placed.
A further object of the invention is to provide intravascular microwave warming apparatus which simultaneously monitors accurately the temperature of the blood during the warming process.
Another object of the invention is to provide microwave apparatus to controlledly ablate or necrose tissue within a patient.
Other objects will, in part, be obvious and will, in part appear hereinafter.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying the features of construction, combination of elements and arrangement of parts which are adapted to effect such steps, all is exemplified in the following detailed description, and the scope of the invention would be indicated in the claims.
Briefly, the intravascular blood warming technique specifically disclosed herein is intended to provide uniform and controlled heating in a hypothermic patient using microwave apparatus which safely and efficiently warms the patient""s blood in order to raise the patient""s body core temperature.
The warming apparatus comprises a relatively long, somewhat flexible intravascular catheter capable of being threaded through a conventional introducer to a major blood vessel such as the superior or inferior vena cava. At its distal end, the catheter incorporates an antenna and an expandable stand-off device which prevents the distal end of the catheter, and more particularly the antenna, from contacting the wall of the blood vessel and potentially overheating tissue at that wall. A single cable extending from the proximal end of the catheter is connected to an extracorporeal control and display unit which supplies power to and receives temperature-indicating signals from the catheter.
The control and display unit includes a microwave transmitter which produces a signal having a suitable heating frequency. That signal is applied by way of a diplexer to the cable leading to the antenna in the catheter. This causes the antenna to emit electromagnetic radiation capable of heating high dielectric/high loss organic material such as blood in the vicinity of the catheter.
Also, connected to the diplexer in the control and display unit is a microwave receiver preferably in the form of a radiometer. As is well known, radiometry is a technique for measuring electromagnetic radiation considered as thermal radiation. The single antenna in the catheter is able to detect the microwave radiation emitted by the material surrounding the catheter and that microwave signal is applied by way of the cable and diplexer to the receiver which produces an electrical signal indicative of the temperature of that material. That signal is applied by way of a processor to a display in the control and display unit which thereupon provides a visible indication of that temperature. That temperature-indicating signal can also be used to enable the processor to control the transmitter so as to effect controlled heating of the material surrounding the catheter. As we shall see also, the control and display unit includes means for detecting whether the aforementioned stand-off device in the catheter is open or closed to ensure that the catheter is in the correct position in the blood vessel before the catheter""s antenna is activated.
As will be described in more detail later, the diplexer in the control and display unit allows for the separation of the relatively low heating frequency of the transmitter from the much higher radiometer frequency. Resultantly, the apparatus can use a common antenna and cable connection to the control and display unit to both transmit (heat) and receive (measure temperature). Thus, the diplexer and associated radiometer eliminate the need for thermocouples or thermistors in the catheter thereby minimizing the cost of, and improving the performance and safety record of, the catheter. It should be emphasized in this connection that this cost and performance comparison is not being made between just a radiometer and a thermocouple, but rather with all of the ancillary parts such as wires, connectors and amplifiers that have to support the thermocouple. Elimination of all of these parts enhances the flexibility of the catheter and greatly improves the overall reliability and maintenance record of the apparatus.
Most importantly, since the present apparatus senses temperature using radiometry, the temperature sensed is the actual temperature of the blood surrounding the catheter rather than the catheter tip temperature as would be the case if the catheter incorporated a thermocouple or thermister for temperature detection and control.
Similar devices incorporating the invention can be used in other applications. In tissue ablation, for example, the configuration of the catheter depends upon the body cavity being accessed. Thus, to treat BPH, a transurethral catheter or probe is used which may incorporate a conventional helical antenna and receive sufficient power to raise the patient""s intraprostatic temperature sufficiently and for a sufficient time to selectively necrose the BPH. Such a catheter typically incorporates a cooling circuit adjacent the antenna to ensure that the patient""s urethra is not heated above a safe temperature, and an expandable balloon to properly position the catheter; see my application Ser. No. 09/476,201, filed Jan. 3, 2000, the contents of which is hereby incorporated herein by reference.
Transurethral microwave heating apparatus incorporating this invention would allow a common antenna in the catheter or probe to provide both heating and measurement of temperature, thereby eliminating the requirement of thermocouples and wires and allowing closer control over actual tissue temperature, as well as providing a positive indication of the condition of the catheter""s expandable device. In this case, the expandable device, when open, would seat against the neck of the patient""s bladder to properly locate the catheter or probe.
When used for myocardial ablation, the present apparatus would include a catheter with an antenna and an expandable stand-off device and capable of being threaded into a patient""s heart muscle in order to controlledly heat heart tissue in order to necrose said tissue. In this application, the expandable device is used to center the catheter.
Other applications for the invention may be envisioned.